Wearable devices such as wrist-worn devices can nowadays comprise a plurality of functions. Examples of such functions are time display, measurement and display of various physical quantities like temperature, air pressure, heart beat rate and so on. A wearable device can also comprise a radio unit for receiving and/or sending radio signals, for instance a GPS receiver (Global Positioning System).
Integrating an antenna into a wearable device comprising a radio unit can be problematic because other electronics in the device, particularly a circuit substrate, such as a printed circuit board, comprising conductive parts, may act as a ground plane for the antenna and deteriorate the performance of the antenna. Also the proximity of a human body can deteriorate the performance of the antenna especially in a wrist-worn device. Furthermore, a wearable device is typically of limited size and on the other hand the antenna should not be very protruding which also constricts the antenna design.
A commonly used antenna type that can be placed inside a wearable device is a so-called patch antenna. A patch antenna typically comprises a planar radiator and a ground plane parallel thereto. A problem with the patch antenna type is the relatively large surface area it has on account of which it can be difficult to place a patch antenna inside a wearable device, especially a small wearable device.
U.S. Pat. No. 4,977,614 describes an antenna incorporated into a wristband of a wrist-worn device. A problem with this kind of antenna design is that the antenna may be strongly coupled to the wrist of a user thus impairing the antenna performance.
U.S. Pat. No. 5,926,144 describes an antenna located within the housing of a wearable electronic device. In the solution presented the antenna is a loop antenna which is positioned vertically with respect to the housing of the device. A problem with this kind of design is that it potentially increases the vertical space needed inside the housing.
U.S. Pat. No. 5,768,217 describes also an antenna located within the housing of a wearable electronic device. In the solution presented the antenna is a loop antenna which is positioned horizontally with respect to the housing of the device on top of the circuit board and the components therein. The potential problem with this kind of design is that especially the circuit board may act as a ground plane for the antenna and affect the performance of the antenna in an undesirable way.